Reproducibility of QT Parameters Derived from 24‐hour Ambulatory ECG Recordings in Healthy Subjects

Aim: To estimate the reproducibility of QT parameters derived from 24‐hour ambulatory ECG recordings. Method: Ten healthy volunteers aged 25 to 41 years participated. In two 24‐hour ambulatory ECG recordings obtained 1 day apart, the QT interval was measured manually at stable heart rates in approximately 16 periods during daytime and 6 periods during nighttime. The association between the QT and RR interval was described by linear regression for day and nighttime separately and the following QT parameters were calculated: the QT interval at heart rate 60 beats/min during daytime (QT(60)day), slope(day), slope(night), and the difference in QT(60) between day and nighttime (ΔQT(60)). The QT parameters were assessed four times for each participant to discriminate method inaccuracy from day to day variation. The reproducibility was estimated as the coefficient of repeatability, the relative error, and the ratio between within‐subject variability and between‐subject variability. Results: The coefficient of repeatability, the relative error and the ratio, respectively, were 19 ms, 1.8% and 0.5 for QT(60)day, 0.076, 21% and 0.68 for slope(day), 0.116, 43% and 1.37 for slope(night), and 37 ms, 325% and 1.19 for ΔQT(60) when estimating the overall day to day reproducibility. Inaccuracy of QT measurement accounted for approximately 40% of this variation, whereas the error caused by selecting segments was small. Conclusion: QT(60)day has a high reproducibility and may with advantage replace the conventional QT interval measured on a resting ECG. To assess QT dynamics, the slope of the regression line during daytime is suitable and the short term reproducibility acceptable for clinical trials. Regarding slope(night) and ΔQT(60), the variation is high and the parameters should be used with caution. A.N.E. 2001;6(1):24–31     

Evaluation of QT Interval Dispersion in a Multiple Electrodes Recording System versus 12‐Lead Standard ECG in an In Vitro Model

Background: QT interval dispersion (QTID) as assessed on conventional surface electrocardiogram (ECG) has been used as a clinical tool to identify patients at high risk of ventricular arrhythmia. However, the results obtained have been controversial. The main purpose of this study was to compare QTID measured from an array of 5 × 6 electrodes homogeneously distributed against the values found when the 12‐lead standard ECG was used. Methods: QTID was calculated in a modified Langendorff‐perfused rabbit heart model immersed in a cylindrical chamber. Dispersion in ventricular repolarization was artificially increased by d‐sotalol (60 μ;m) perfusion. Results: All the duration variables measured from any of the lead systems used were significantly increased after d‐sotalol perfusion. The most commonly used variables in clinical studies, such as QTID (maximum ‐ minimum), do not reach a level of statistical significance, except when measured from the 30‐electrodes array or 15 electrodes covering the left or right side of the heart. The standard deviation of the QT interval (QTI) hardly reached a significant level (P = 0.0499) when calculated from the 12‐lead standard ECG. QTID measured at the peak of the T wave exhibited the highest level of significance when calculated from any of the lead systems used. Conclusion: Thirty electrodes homogeneously distributed on the body surface can better discriminate changes in heterogeneity of repolarization. These data further support the importance of multiple recording systems for the evaluation of QTID and may help to provide an understanding of the discrepancies found in clinical applications.     

Interobserver Reproducibility of QT Interval Measurement and QT Dispersion in Patients After Acute Myocardial Infarction

Background: The study evaluated interobserver differences in the classification of the T-U wave repolarization pattern, and their influence on the numerical values of manual measurements of QT interval duration and dispersion in standard predischarge 12-lead ECGs recorded in survivors after acute myocardial infarction. Methods: Thirty ECGs recorded at 25 mm/s were measured by six independent observers. The observers used an adopted scheme to classify the repolarization pattern into 1 of 7 categories, based on the appearance of the T wave, and/or the presence of the U wave, and the various extent of fusion between these. In each lead with measurable QRST(U) pattern, the RR, QJ, QT-end, QT-nadir (i.e., interval between Q onset and the nadir or transition between T and U wave) and QU interval were measured, when applicable. Based on these measurements, the mean RR interval, the maximum, minimum, and mean QJ interval, QT-end and/or QT-nadir interval, and QU interval, the difference between the maximum and minimum QT interval (QT dispersion [QTD]), and the coefficient of variation of QT intervals was derived for each recording. The agreement of an individual observer with other observers in the selection of a given repolarization pattern were investigated by an agreement index, and the general reproducibility of repolarization pattern classification was evaluated by the reproducibility index. The interobserver agreement of numerical measurements was assessed by relative errors. To assess the general interobserver reproducibility of a given numerical measurement, the coefficient of variance of the values provided by all observers was computed for each ECG. Statistical comparison of these coefficients was performed using a standard sign test. Results: The results demonstrated the existence of remarkable differences in the selection of classification patterns of repolarization among the observers. More importantly, these differences were mainly related to the presence of more complex patterns of repolarization and contributed to poor interobserver reproducibility of QTD parameters in all 12 leads and in the precordial leads (relative error of 31%–35% and 34%–43%, respectively) as compared with the interobserver reproducibility of both QT and QU interval duration measurements (relative error of 3%–6%, P < 0.01). This observation was not explained by differences in the numerical order between QT interval duration and QTD, as the reproducibility of the QJ interval (i.e., interval of the same numerical order as QTD was significantly better (relative error of 7.5%–13%, P < 0.01) than that of QTD. Conclusions: Poor interobserver reproducibility of QT dispersion related to the presence of complex repolarization patterns may explain, to some extent, a spectrum of QT dispersion values reported in different clinical studies and may limit the clinical utility in this parameter.     

Reproducibility and automatic measurement of QT dispersion

This study investigated interobserver (two observers) and intrasubject(two measurements) reproducibility of QT dispersion from abnormalelectrocardiograms in patients with previous myocardial infarction,and compared a user-interactive with an automatic measurementsystem. Standard 12-lead electrocardiograms, recorded at 25mm. s^–1, were randomly chosen from 70 patients followingmyocardial infarction. These were scanned into a personal computer,and specially designed software skeletonized and joined eachimage. The images were then available for user-interactive (mouseand computer screen), or automatic measurements using a speciallydesigned algorithm. For all methods reproducibility of the RRinterval was excellent (mean absolute errors 3–4 ms, relativeerrors 0·3–0·5%). Reproducibility of themean QT interval was good; intrasubject error was 6 ms (relativeerror 1·4%), interobserver error was 7 ms (1·8%),and observers' vs automatic measurement errors were 10 and 11ms (2·5, 2·8%). However QTc dispersion measurementshad large errors for all methods; intrasubject error was 12ms (17·3%), interobserver error was 15 ms (22·1%),and observers' vs automatic measurement were errors 30 and 28ms (35·4, 31·9%). QT dispersion measurements relyon the most difficult to measure QT intervals, resulting ina problem of reproducibility. Any automatic system must notonly recognize common T wave morphologies, but also these moredifficult T waves, if it is to be useful for measuring QT dispersion.The poor reproducibility of QT dispersion limits its role asa useful clinical tool, particularly as a predictor of events.     

QT Intervals and QT Dispersion Determined from a 12‐Lead 24‐Hour Holter Recording in Patients with Coronary Artery Disease and Patients with Heart Failure

Background: QT dispersion is considered an index of spatial inhomogeneity of repolarization duration and increased dispersion of ventricular repolarization is supposed to increase the risk of ventricular arrhythmia. Circadian variation in QT dispersion was investigated. Methods: Three different modes of lead selection was used: all 12‐leads (QTdisp 12), only precordial leads (QTdisp 6), and one pair of preselected leads (QTdisp 2) in a 24‐hour Holter recording every fourth hour each comprising 10 consecutive measurements in 54 healthy subjects, 29 patients with coronary artery disease (CAD), and 29 patients with heart failure (HF). Results: A significant circadian variation was observed in healthy subjects when modes QTdisp 12 and QTdisp 6 were used (Mean ± SD 35.58 ± 16.48 ms; P < 0.0001; and 28.82 ± 16.02 ms; P < 0.0001, respectively), and in patients with CAD (Mean ± SD 37.86 ± 17.87 ms; P < 0.01; and 28.72 ± 17.06 ms; P < 0.0001, respectively), whereas no circadian variation was observed in QTdisp 2. No circadian variation was observed in patients with HF irrespectively of lead selection. Patients with CAD without myocardial infarction (MI) had a circadian variation in QTdisp 12 (Mean ± SD 33.13 ± 14.86 ms; P < 0.05), whereas no circadian variation was observed in patients with MI (Mean ± SD 40.35 ± 18.80 ms; P = NS). Conclusions: Circadian variation of QT dispersion was detected in healthy subjects and in patients with uncomplicated CAD, but not in those who had suffered a previous MI and in patients with HF. The number of leads among which selection of the longest and shortest QT intervals took place was critical for the disclosure of circadian variation of QT dispersion.     

Weighing the QT Intervals with the Slope or the Amplitude of the T Wave

Objective: The reproducibility of QT interval measurements is low, even for the mean QT interval based on the standard ECG. In this study we analyzed whether the reproducibility of the mean weighed QT interval was better than the simple mean QT interval. The weighing was based on the amplitude of the T wave or the slope of the steepest tangent on the terminal part of the T wave. Material and methods: 12‐lead ECGs of 130 postmyocardial infarction patients were obtained. The QT intervals were measured by the tangent‐method on two occasions by the same observer Mismatch QT intervals were defined as QT intervals that were measured at only one occasion. Sixteen ECGs were rejected. The data were split into 34 and 80 ECGs for optimization and validation of the weighing, respectively. The weighed QT dispersion was calculated as the weighed mean of the three longest minus the weighed mean of the three shortest QT intervals. Results: Weighing with the slope increased the reproducibility by 41% (P = 3 10^‐6), but weighing with the amplitude reduced it by 20% (P = 0.02). However, if measurements with errors above 75 ms were rejected, weighing with the slope or the amplitude increased the reproducibility with 26% and 20% (P = 0.02), respectively. Weighing did not change the reproducibility of the weighed QT dispersion. Conclusion: Weighing with the slope improved the reproducibility of the mean weighed QT interval. However, if measurements with errors above 75 ms were rejected, weighing with the amplitude also increased the reproducibility. Weighing did not change the reproducibility of the weighed QT dispersion. Weighing is particularly efficient at reducing the negative impact of mismatch QT intervals on the reproducibility. A.N.E. 2002;7(1):4–9     

Spatial Properties of QT and the T‐Wave Morphology

Objective: To describe the relation between the QT interval and the T‐wave morphology. Material and methods: Frank orthogonal leads X, Y, Z of one subject and resting 12‐lead ECG of 40 subjects. QT was measured by the tangent method. The QT values are organized according to the anatomic orientation of the leads: I, ‐aVR, II, aVF, III, ‐aVL, ‐I, aVR, ‐II, ‐aVF, ‐III, aVL. and: V₁, V₂, V₃, V₄, V₅, V₆, ‐V₁ ‐V₂, ‐V₃, ‐V₄, ‐V₅, ‐V₆. The T‐wave amplitudes and QT were categorized according to QT into four groups with increasing mean QT. Results: Kruskal‐Wallis nonparametric test showed that the shortest and longest QT values are measured on the T wave with the smallest amplitudes (P < 0.001). Inspection of plots of QT and T waves reveals that the shortest and longest QT values are usually measured in leads with a small difference in orientation (neighbor leads). The mechanism behind these characteristics is mainly that the shortest and longest QT values are measured on T waves that are close to a lead orientation, whereas the T waves are flat or biphasic. We also observed an almost significant (P = 0.057) decrease in the T‐wave amplitude with increasing dispersion. Conclusion: The relation between T‐wave morphology and QT in the same cardiac plane is highly organized. The shortest and longest QT values are measured on the T wave with the smallest amplitudes (P < 0.001).     

Bias of QT Dispersion

Background: Prolonged QT dispersion (QID) is associated with an increased risk of arrhythmic death but its accuracy varies substantially between otherwise similar studies. This study describes a new type of bias that can explain some of these differences. Material: One dataset (DiaSet) consisted of 356 subjects: 169 with diabetes, 187 nondiabetic control persons. Another dataset (ArrSet) consisted of 110 subjects with remote myocardial infarction: 55 with no history of arrhythmia and 55 with a recent history of ventricular tachycardia or fibrillation. Methods: 12‐lead surface ECGs were recorded with an amplification of 10 mm/mV at a paper speed of 50 mm/s. The QT interval was measured manually by the tangent‐method. The bias depends on the magnitude of the measurement errors and the measurable part of the bias increases with the number of the repeated measurements of QT. Results: The measurable bias was significant for both datasets and decreased for increasing QTD in the DiaSet (P < 0.001) and in the ArrSet (P = 0.11). The bias was 2.5 ms and 1.9 ms at QTD = 38 ms and 68 ms, respectively, in the ArrSet, and 7.5 ms and 2.8 ms at QTD = 19 ms and 55 ms, respectively, in the DiaSet. Conclusions: This study shows that random measurement errors of QT introduces a type of bias in QTD that decreases as the dispersion increases, thus reducing the separation between patients with low versus high dispersion. The bias can also explain some of the differences in the mean QTD between studies of healthy populations. Averaging QT over three successive beats reduces the bias efficiently. A.N.E. 2001;6(1):38–42     

自发Ⅰ型Brugada波QT间期及QT离散度变化的意义

目的 了解自发Ⅰ型Brugada波患者的QT间期及QT间期离散度变化的意义.方法 对15例自发Ⅰ型Brugada波患者进行12导联心电图检测,描记12导联同步心电图,测量QT间期,QT离散度,比较呈典型Ⅰ型Brugada波与无Brugada波之间的差异及对照组之间的差异.结果 患者自发Ⅰ型Brugada波时,QT间期...     

A Study of the Association between the Prolongation of the QT Interval in the Resting ECG and Myocardial Infarction

ABSTRACT The relationship between the incidence of myocardial infarction in the 10 year follow-up period and the length of the QT interval and its two components (the time elapsing between the Q wave and the beginning of the T wave, and the duration of the T wave) was investigated in a study of the records of a group of men drawn from a random sample of all 55-year-old men living in Göteborg, Sweden. A significant association was found between the incidence of myocardial infarction and the first component but not with the second component or the QT interval itself. The two components were found to be independent and thus to have the potential to act as confounding factors if the QT interval is examined alone. Further, our results suggest that correcting the QT interval for heart rate needs careful reassessment.     

Improving the Reproducibility of QT Dispersion Measures

Background: The low reproducibility of the QT dispersion (QTD) method is a major reason why it is not used in clinics. The purpose of this study was to develop QT dispersion parameters with better reproducibility and identification of patients with a high risk of ventricular arrhythmia or death. Methods and Results: Three institutions using different methods for measuring QT intervals provided QT databases, which included more than 3500 twelve‐lead surface ECGs. The data represented low and high risk subjects from the following groups: the normal population EpiSet (survivors vs dead from cardiovascular causes), acute myocardial infarction patients AmiSet (survivors vs dead) and remote myocardial infarction patients ArrSet (with vs without a history of ventricular arrhythmia). The EpiSet, AmiSet, and the ArrSet contributed with N = 122, 0, and 110 ECGs for reproducibility analysis, and 3244, 446, and 100 ECGs for the analysis of prognostic accuracy. The prognostic accuracy was measured as the area under the Receiver Operator Curve. The QT intervals were divided into six QT pairs; the longest pair consisted of the longest and the shortest QT intervals etc. The QT dispersion trend (QTDT) was defined as the slope of the linear regression of the N longest QT pairs after estimation of missing QT intervals by interpolation of measured QT intervals. The QTMAD and the QTSTD methods were defined as twice the mean absolute deviation and the standard deviation of the N longest QT pairs. The reproducibility was improved by 27% and 19% in the EpiSet and the ArrSet relative to the reproducibility of QTD. The accuracy improved for the EpiSet and the ArrSet and was maintained for the AmiSet. Conclusions: By using the three longest and the three shortest QT intervals in QTDT, QTMAD, or QTSTD, the reproducibility improved significantly while maintaining or improving the prognostic accuracy compared to QTD. A.N.E. 2001;6(2):143–152     

QT Dispersion in Healthy Chinese Children and Adolescents

Background: QT dispersion (QT_d) reflects the interlead difference in QT interval. It may provide a measure of repolarization inhomogeneity. Studies on QT_d mainly involve adults, while QT_d in children are less well studied. The aim of this study was to evaluate QT_d in healthy children and assess the relationship of gender, age, and anthropometric parameters, viz. weight (W), height (H), body mass index (BMI), and body surface area (BSA) to QT_d. Methods: Five hundred and one Chinese children and adolescents (243 boys, 258 girls) with no history of cardiovascular diseases were studied. Their ages ranged from 6.3 to 17.5 years. Surface 12-lead electrocardiograms were measured in each child at rest. QT and R-R intervals in each of the 12 leads were manually measured at a magnification of 2X. QT was corrected to QT_c according to Bazett's formula. QT_d was calculated as the difference between the maximum and minimum QT of the measured leads, while corrected QT_d (QT_cd) was the difference between the maximum and minimum QT_c of the measured leads. Adjusted QT_d was QT_cd divided by the square root of the number of measurable leads. Results: Mean QT_d of all subjects was 34 ms (95% Cl 33.6–35.1 msl. Mean QT_d for boys and girls was 35 ms and 34 ms, respectively (P = 0.18). Mean QT_cd for the whole group was 47 ms (95% Ci 45.8–48.2 ms), while mean adjusted QT_cd was 14 ms (95% Cl 13.8–14.5 ms). There were no significant gender differences in QT_cd or adjusted QT_cd. Weak negative correlation existed between age and QT_d, QT_d and adjusted QT_cd (r =?0.22, r =?0.26, r =?0.21, respectively, P < 0.001 Similarly, QT_cd also had a weak significant negative correlation with W (r =?0.20), H (r =?0.21) and BSA (r =?0.22), P < 0.001. However, multiple stepwise regression analysis revealed that only age was significantly related to QT_cd (R2 = 0.066) and QT_d (R2 = 0.059), P < 0.001. Conclusions: The results of this study indicate a trend of decreasing QT_d and QT_cd with increasing age, supported by multiple regression analysis. However indices of QT_d in children are not influenced by anthropometry. This information may be useful for the clinical application of QT_d in children. A.N.E. 1999;4(3):281–285     

对QT离散度实质的探讨

为探讨ＱＴ离散度（ＱＴｄ）的真实意义，观察１３９例急性心肌梗死（ＡＭＩ，ＡＭＩ组）及１０９例正常人（对照组）的最长ＱＴ间期（ＱＴｍａｘ）、校正ＱＴｍａｘ（ＱＴｃｍａｘ）及ＱＴｄ的变化。结果：①ＡＭＩ组的ＱＴｍａｘ、ＱＴｃｍａｘ和ＱＴｄ均显著高于对照组（分别为４２２．６０±３０．５１ｍｓｖｓ３８２．４６±２３．４０ｍｓ、４６０．２１±２８．９６ｍｓｖｓ３８８．５１±２０．１５ｍｓ、５９．８０±２８．４０ｍｓｖｓ３９．４３±１２．２１ｍｓ，Ｐ均＜０．００１）。②ＡＭＩ组中发生严重室性心律失常（ＶＡ）患者（１１４例）的ＱＴｍａｘ、ＱＴｃｍａｘ、ＱＴｄ与无ＶＡ的患者（２５例）相比，均有显著差异（分别为４４８．５８±３３．４０ｍｓｖｓ４１６．１０±３５．３０ｍｓ、４８１．４３±３５．１７ｍｓｖｓ４３９．６０±２７．１０ｍｓ、６６．９０±２０．７２ｍｓｖｓ４８．３２±２３．６１ｍｓ，Ｐ均＜０．００１）。认为ＡＭＩ时ＱＴｄ系Ｔ向量环在不同导联上的“投影”差异所引起的，其异常的本质是ＱＴ间期延长     

QT interval in cardiac amyloidosis

The aim of the study was to investigate whether cardiac amyloidosis is associated with QT interval abnormalities and ventricular arrhythmias. A controlled study of 30 patients was undertaken at a university cardiology department in a large referral hospital. Thirty patients (18 men, 12 women, mean age 56 ± 12 years) with systemic amyloidosis verified by biopsy and strong indications of cardiac amyloidosis comprised the study group, with 30 healthy age- and sex-matched individuals serving as controls. Complete M-mode and two-dimensional echocardiographic study was undertaken and QT interval and QT_c were calculated. All patients and controls underwent 24-h Holter monitoring for arrhythmias. Left ventricular (LV) wall thickening was found in all patients with cardiac amyloidosis. The LV mass in the patients with cardiac amyloidosis was significantly greater than that of the control group, as was the ratio LVmass/body surface area (p < 0.001). There was no significant difference in the max QT interval or in QT_c dispersion between the two groups, although the max QT_c was greater in the patients with cardiac amyloidosis. Patients with cardiac amyloidosis did not have a higher incidence of arrhythmias than the controls. Although patients with thickened cardiac walls due to cardiac amyloidosis have a prolonged QT_c in comparison with controls, they do not show an increase in interlead QT_c dispersion which might suggest the possibility of regional disturbances of the uniformity of repolarization. Patients with cardiac amyloidosis do not have a higher incidence of arrhythmias than controls.     

The prognostic value of the QT interval and QT interval dispersion in all-cause and cardiac mortality and morbidity in a population of Danish citizens

Aims. To evaluate the prognostic value of the QT interval and QT intervaldispersion in total and in cardiovascular mortality, as wellas in cardiac morbidity, in a general population. Methods and results. The QT interval was measured in all leads from a standard 12-leadECG in a random sample of 1658 women and 1797 men aged 30–60years. QT interval dispersion was calculated from the maximaldifference between QT intervals in any two leads. All causemortality over 13 years, and cardiovascular mortality as wellas cardiac morbidity over 11 years, were the main outcome parameters.Subjects with a prolonged QT interval (430ms or more) or prolongedQT interval dispersion (80ms or more) were at higher risk ofcardiovascular death and cardiac morbidity than subjects whoseQT interval was less than 360ms, or whose QT interval dispersionwas less than 30ms. Cardiovascular death relative risk ratios,adjusted for age, gender, myocardial infarct, angina pectoris,diabetes mellitus, arterial hypertension, smoking habits, serumcholesterol level, and heart rate were 2·9 for the QTinterval (95% confidence interval 1·1–7·8)and 4·4 for QT interval dispersion (95% confidence interval1·0–19·1). Fatal and non-fatal cardiac morbidityrelative risk ratios were similar, at 2·7 (95% confidenceinterval 1·4–5·5) for the QT interval and2·2 (95% confidence interval 1·1–4·0)for QT interval dispersion. Conclusion. Prolongation of the QT interval and QT interval dispersion independentlyaffected the prognosis of cardiovascular mortality and cardiacfatal and non-fatal morbidity in a general population over 11years.     

肺动脉高压患者心率校正QT间期和QT离散度的研究

目的:检测肺动脉高压(pulmonary hypertension,PH)患者的心率校正的QT间期(heartrate-corrected QT interval,QTc)和QTc离散度(QTc dispersion,QTcd),并评价其与肺动脉压力的关系。方法:入选2003年12月至2008年7月因初步诊断为PH而进行右心导管术的患者。记录静息12导联心电图,手工测量QT间期并用Bazett公式进行校正。根据平均肺动脉压,将患者分为对照组,轻-中度PH组和重度PH组。结果:共入选201例患者。男性患者的QTc和QTcd在3组间差异无统计学意义。女性患者中,重度PH组的QTc比对照组高〔(436.1±39.4)msvs.(407.6±24.8)ms,P=0.037〕,重度PH组的QTcd(68.5±20.9)ms高于对照组(45.1±12.6)ms和轻-中度组(58.6±14.7)ms(P=0.002;P=0.003)。此外,女性患者的QTc和QTcd与平均肺动脉压正相关(r=0.207,P=0.03;r=0.236,P=0.012)。结论:本组资料中女性PH患者的QTc和QTcd与平均肺动脉压正相关,且在重度PH患者中显著增高,有待于进一步探讨。